The present invention relates to a magnetic recording unit and, more particularly, to a magnetic recording medium and a method for production thereof.
Among magnetic recording medium proposed so far are one with artificially patterned recording tracks (discrete track medium “DTM”) and one with patterned recording bits (patterned medium “PM” or discrete bit medium “DBM”). (See Japanese Patent No. 188363 and Japanese Patent Laid-open Nos. Hei-6-231443 and Hei-4-336404, for example.) For their increased recording density, the PM and DTM require microfabrication of submicron order for the recessed-relieved structure corresponding to the bits and tracks. Such a microstructure is to be formed by means of electron beam lithography “EBL” (as disclosed in R. M. H. New et al., JVST B 12 (1994), p. 3196) or X-ray lithography (as disclosed in F. Rousseaux et al., JVST B 13 (1995), p. 2787). Unfortunately, EBL and X ray lithography cause high production cost.
One way to produce the microstructure of submicron order efficiently at a low cost is by imprinting to form micropatterns (as disclosed in S. Y. Chou et al., SCIENCE 272 (1996), p. 85). Imprinting is designed to bring a preformed mold into close contact with a coated substrate, thereby replicating the recessed-relieved structure of the mold in the coating on the substrate. Imprinting includes several methods as listed below.
Thermal imprinting that employs a thermoplastic resin as the transferred body. The process consists of softening the thermoplastic resin on a substrate by heating above its glass transition temperature (Tg), pressing a mold against the softened resin, cooling the mold and resin, and removing the cured resin.
Photo-imprinting that employs a photo-curable resin as the transferred body. The process consists of pressing a mold against the photo-curable resin coated on a substrate and irradiating the resin with light to achieve photocuring.
Chemical imprinting that employs as the transferred body a resin that softens in a specific atmosphere.
All of the methods mentioned above involve the steps of pressing a mold against a resin as the transferred body, curing the resin, and removing the mold, thereby replicating the recessed-relieved structure of the mold in the transferred body. According to U.S. Pat. No. 5,772,905, the recessed-relieved structure replicated in the transferred body on the substrate is used as a mask for the etching of the substrate.
Production of magnetic recording medium by the above-mentioned imprinting technology is reported in S. Y. Chou et al., JAP 79 (1996), p. 6101 (for PM) and D. Wachenschwanz et al., JAP 79 (1996), p. 6101 (for DTM). Production of magnetic recording medium by liftoff is reported also in S. Y. Chou et al., JAP 79 (1996), p. 6101. There is disclosed another method for producing a magnetic recording medium by forming a resist structure on a flat magnetic film and etching the magnetic film through the resist structure as a mask. See K. Hattori et al., IEEE Trans. Magn. 40 (2004), p. 2510. What is common to the technologies mentioned above is that the transfer resist used as the mask is removed eventually.
U.S. Pat. Nos. 6,809,356 and 6,518,189 disclose a technology for deforming a resist film by imprinting with heat and pressure and using the deformed resist film as a CD for high-density recording. Japanese Patent Laid-open No. 2003-178431 discloses a method for producing a magnetic recording medium by coating a soft magnetic layer with a thermoplastic material, patterning the thermoplastic material with heat and pressure, and embedding the magnetic layer in recesses formed by patterning. Japanese Patent Laid-open No. 2004-62995 discloses a substrate for a magnetic recording medium, said substrate containing a thermoplastic resin in which are formed recessed-relieved patterns. Japanese Patent Laid-open Nos. Sho-59-124030, Sho-60-195738, and Sho-63-222349 disclose a magnetic recording medium or opto-magnetic recording medium having the recessed-relived structure formed thereon. Japanese Patent Laid-open No. Sho-62-169643 discloses a vinyl ester that is used directly as the substrate of optical recording medium.
Making the microstructure in magnetic recording medium by means of electron beam lithography or X ray lithography involves difficulties in mass production because of its high cost. The above-mentioned imprinting technology to make the recessed-relieved structure by means of heat and pressure is applicable only to thermoplastic resins. The CD is a medium for replay only and hence its technology is not applied to magnetic recording medium for recording and reproduction. In other words, the CD technology does not assume piled magnetic recording layers having magnetic properties necessary for the recessed-relieved structure. The technologies disclosed in Japanese Patent Laid-open Nos. 2003-178431 and 2004-62995 do not permit reheating the thermoplastic resin (beyond its melting point or softening point) once the recessed-relieved structure has been formed thereon. The methods disclosed in Japanese Patent Laid-open Nos. Sho-59-124030, Sho-60-195738, and Sho-63-222349 are intended to prevent adherence, and they are not designed to make the magnetic structure for PM and DTM. The optical or magneto-optical recording disclosed in Japanese Patent Laid-open No. Sho-62-169643 is limited less than magnetic recording in the flatness and hardness of the substrate because it permits a larger head flying height. Although magnetic recording media are greatly affected by the flatness and hardness of the substrate, the disclosed material and system do not provide the satisfactory flatness and hardness required of them and hence they cannot be applied to the substrate for magnetic recording medium. Moreover, the disclosed material is limited in heat resistance and hence in heat treatment temperature (60 to 200° C.); therefore, it is not suitable for the magnetic recording medium that need heat treatment at high temperatures for their specific magnetic properties.